Co-electrolysis of seawater and carbon dioxide inside a microfluidic reactor to synthesize speciality organics

We report co-electrolysis of seawater and carbon dioxide (CO2) gas in a solar cell-integrated membraneless microfluidic reactor for continuous synthesis of organic products. The microfluidic reactor was fabricated using polydimethylsiloxane substrate comprising of a central microchannel with a pair of inlets for injection of CO2 gas and seawater and an outlet for removal of organic products. A pair of copper electrodes were inserted into microchannel to ensure its direct interaction with incoming CO2 gas and seawater as they pass into the microchannel. The coupling of solar cell panels with electrodes generated a high-intensity electrical field across the electrodes at low voltage, which facilitated the co-electrolysis of CO2 and seawater. The paired electrolysis of CO2 gas and seawater produced a range of industrially important organics under influence of solar cell-mediated external electric field. The, as synthesized, organic compounds were collected downstream and identified using characterization techniques. Furthermore, the probable underlying electrochemical reaction mechanisms near the electrodes were proposed for synthesis of organic products. The inclusion of greenhouse CO2 gas as reactant, seawater as electrolyte, and solar energy as an inexpensive electric source for co-electrolysis initiation makes the microreactor a low-cost and sustainable alternative for CO2 sequestration and synthesis of organic compounds.

www.nature.com/scientificreports/ carboxylic acid, glycol, and carboxylate compounds by simultaneously electrolyzing H 2 O in one compartment while reducing CO 2 in another compartment [31][32][33][34][35] . It has been demonstrated that co-electrolysis of CO 2 and H 2 O with alternative energy sources such as wind and solar irradiation can be used to produce hydrocarbon fuels and industrial chemicals [36][37][38] . Literature reports have shown that microchannel reactors can reduce CO 2 under galvanic conditions by catalytic electrochemical reduction 39,40 . An efficient conversion of greenhouse CO 2 gas into value-added hydrocarbons would be feasible with the development of a catalyst-free solar-powered microreactor system. Herein, we report a portable, self-reliant, and low-cost polydimethylsiloxane (PDMS) microreactor for continuously producing a variety of commercially significant organic compounds in a sustainable manner using naturally abundant CO 2 , seawater, and solar energy. A proof-of-concept membraneless microreactor for CO 2 utilisation is developed with the following components-(i) an integrated T-shaped microchannel with a photovoltaic cell and conductive contacts to produce a high-intensity electrical field inside the microchannel which will allow CO 2 to be chemically converted under solar radiation, (ii) two inlets-one for incoming CO 2 gas and one for the inflow of seawater, and an outlet for collecting organic compounds. Since the integrated solar panels convert solar illumination into electrical energy, the proposed microreactor requires low operating electrical power for chemical reactions. The embedded electrodes, separated by a microscale gap, generate a high intensity field within the microchannel reactor at lower potential differences. When CO 2 and seawater are introduced into the microreactor, the gas-liquid mixture encounters the high intensity electrical field inside the microreactor, and undergoes rapid co-electrolysis to produce nascent hydrogen (H 2 ) and oxygen (O 2 ) as well as free radicals or ions. By using solar illumination, a variety of organic compounds can be produced, including aldehydes, formate salts, formic acid, primary and secondary alcohols, and hydrocarbons. The microreactor mimics plant photosynthesis by converting CO 2 gas and water into organic products 41 . The novelty of the work lies in the fact that it is arguably the first research in which a catalyst-free PV cell-integrated microreactor has been employed to transform solar energy into electricity and then, with the help of generated electricity, a gas-liquid mixture of CO 2 and seawater has been turned into an array of organic products. Importantly, further investigations suggest that the reaction rate and the organic compound to be synthesized inside the microreactor can be tuned by regulating the electric field strength produced by the PV cells under solar irradiation.
In addition, Fourier-transform infrared (FT-IR) spectroscopy, gas chromatography-mass spectrometry (GC-MS), and high-performance liquid chromatography (HPLC) methods were used to analyse and categorize the organic products obtained from the microreactor, and to understand how these organic products are formed by high intensity electrical fields within the reactor. The assembly of these microreactor prototypes can be used to intensify traditional CO 2 utilization processes in the near future due to their high energy efficiency and low cost of operation. In brief, the proposed microreactor opens up a promising avenue for CO 2 sequestration and clean energy, which may have far-reaching implications for the mitigating global warming.

Results
The sequential steps for fabrication process and the dimension details of the membraneless PDMS-based microreactor can be found in Fig. S1 in the electronic supplementary information (ESI). The Fig. 1 shows the experimental set-up of the microreactor employed for the continuous production of diverse organic compounds from greenhouse CO 2 gas, seawater, and natural sunlight. The microreactor consists of a T-shaped microchannel, two The microreactor has two inlets perpendicular to each other, one for seawater connected to the syringe pump operating at a constant flow-rate (Q w = 3 mL/min) and another for injection of gaseous CO 2 from a pure CO 2 gas cylinder with a mass flow meter operating at a constant flow-rate (Q g = 3 mL/min). The integrated Cu electrodes are positioned perpendicular to the gas-liquid flow and connected to a solar panel circuitry. The microreactor operates under sunlight and the organic products are collected at downstream of microchannel. www.nature.com/scientificreports/ inlets for simultaneous injection of an uninterrupted supply of CO 2 and seawater along with integrated copper (Cu) electrodes for the generation of an electric field for the chemical processing of CO 2 under solar irradiation. The mass flow rate of CO 2 gas (Q g ) and seawater (Q w ) were maintained at a constant rate of constant rate of 3 mL/ min for all the experiments, unless stated otherwise. The two electrodes, positioned several microns apart, were mounted perpendicular to the incoming gas-liquid flow and attached to the outer solar panel to transform solar energy into electrical energy. Under solar irradiation, the closely spaced electrodes produced a strong localized electrical field, thereby, resulting in generation of a high electric potential gradient (ψ) within the microchannel. The gas-liquid mixture of CO 2 and seawater were flown through the microfluidic reactor to produce various organic chemicals such as formate salts, formic acid, primary or secondary alcohols, hydrocarbons along with hydrogen and oxygen under direct solar illumination. Importantly, the rate and composition of as-synthesized organic products were adjusted by regulating the field strength across the electrodes, which, in turn, was controlled by the adjusting the number of solar grid cells exposed to direct sunlight. The electric field potential (ψ) inside the microreactor was varied from 2.5 V to 3.5 V for catalyst-free rapid co-electrolysis of seawater and CO 2 gas and the generated chemical products were collected and analyzed by various characterization techniques.
Proposed electrode-reaction mechanisms. Reaction at cathode. The Fig. 2 shows the proposed electrode mechanism at the cathodic region of the microreactor. The (c) the CO 2 *− species combines with liberated H 2 gas to yield formic acid (CO 2 *− + H 2 → CO 2 H 2 ) as shown in Fig. 2 IC, furthermore the generated formic acid molecules combines with sodium (Na + ) ions present in salty seawater to form formate salts (CO 2 H 2 + Na + → HCO 2 Na). The Fig. 2IIA-C shows the conversion of the aldehydes into alcohols via nucleophilic addition reaction, as aldehydes were more liable to the nucleophilic addition because of their trigonal planar geometry. In such reactions, the nucleophiles used their electron pairs to form a bond with the carbonyl carbon atom. As this happened, the electron pair of carbon-oxygen bond loosened out towards electronegative carbonyl oxygen atom and hybridization state of carbon and oxygen changes from sp 2 to sp 3 hybridization. In the second step, the oxygen atom accepts a proton, resulting in conversion of aldehydes to alcohols. The FT-IR studies. The Fig. 4A shows the FT-IR spectra of the synthesized organic products ranging from formate salts, formic acid, primary or secondary alcohols, to hydrocarbons, collected downstream from microreactor when an electric potential (ψ) of 3.5 V was applied across the electrodes, and a steady flow rate of 3 mL/ min for seawater (Q w ) and CO 2 (Q g ) was maintained within the microchannel. The aqueous solution collected from the microreactor was vacuum-dried to yield powdery sample for FT-IR analysis. The details of the sample preparation for FT-IR analysis have been mentioned in the ESI. The peaks between 3584 and 3230 cm −1 show the formation of alcohols, phenols, and peak at 2924-2859 cm −1 suggests the generation of alkanes, ether, aldehydes, while the peak between 1430 and 1400 cm −1 depicts the production of esters 42 . In order to study the effect of the electrical energy produced by the solar panel on the synthesis of organic compounds, the electrical potential (ψ) across the integrated electrodes inside the microreactor was varied. The Fig. 4B shows the normalized spectra of organic compounds obtained from microreactor at applied electric potential (ψ) values of 2.5 V, 3 V, and 3.5 V and at Q w = 3 ml/min and Q g = 3 ml/min, respectively, thus, confirming that the product distribution depends on the applied potential (ψ). The recorded FT-IR spectral signatures clearly indicate the formation of an assemble of organic compounds (Table 1) such as formate salts, formic acid, primary or secondary alcohols, and hydrocarbons. The FT-IR spectra confirmed the presence of secondary alcohols, higher esters, primary alcohol, esters of aromatic acids, and non-conjugated alkenes in aqueous product sample 42  www.nature.com/scientificreports/ GC-MS and GC studies. The aqueous end-product comprising of all the reaction intermediates was collected in a closed vial from the microreactor outlet and was subjected to GC-MS analysis. The details of the defined parameters for GC-MS analysis, such as lock peak width, peak width (sec.), slope sensitivity (SN), tangent %, peak size reject (counts), and smoothing parameter, has been mentioned in the Table S1 of ESI. The chemical species were analyzed and the corresponding data was validated in the NIST-2010 database in order to classify the organic compounds as per their composition. The identified compounds were confirmed by retention time (min.), probable compound name, area %, molecular weight, as summarized in Table 2. The www.nature.com/scientificreports/ GC-MS confirms the formation of organic products like formate salts, formic acid, primary or secondary alcohols, and hydrocarbons, which correlates with the FT-IR results. For example, the alkenes were detected at retention time of ~ 7.771 min, while the ester were observed at retention time of ~26.514 min in the GC-MS studies, and these two functional groups compounds were also reported in the Fig. 4 and Table 1 of the FT-IR analysis. The generation of the other organic compounds such as secondary alcohols, higher esters, primary alcohol, alkane, and alcohols were verified in the similar manner. The Tables 1 and 2 together confirmed the synthesis of a large number of organic products from the electrochemical reaction between sea water and CO 2 within the microreactor under the influence of applied electric potential gradient. The gaseous products of the reaction were obtained from the closed vial with the aid of a 100 μL gastight syringe and inserted into the GC TCD port for identification. Two distinct peaks were detected in the GC analysis, one with retention time (t R ) of ~ 0.72 min and another with ~ 1.83 min represented H 2 and O 2 , respectively. Furthermore, in order to quantify the amount of H 2 and O 2 gas produced from the reaction, initially, a calibration curve for pure H 2 gas was obtained using a GC instrument. In the calibration method, different volumes of pure H 2 and O 2 gases (0.1-0.9 ml) were collected in 100 μL gastight syringe before injecting into the TCD port of the GC. Thereafter, for a known volume of H 2 and O 2 gases, we obtained a typical GC peak in which the t R was in The gaseous products issuing out of the microreactor were analyzed in GC under same condition using the correlations obtained from the calibration plot as shown in the Fig. S2 and Fig. S3 of the ESI. Interestingly, the GC plots reported that the mixture of gases collected from the micro-reactor was oxy-hydrogen, which was produced by rapid co-electrolysis of CO 2 and seawater at very high electrical field strength within the microchannel near the electrode area. The Fig. 5 shows that when the applied potential values (ψ) were gradually increased from 2.5 V, 3.0 V to 3.5 V, the peak intensities of H 2 and O 2 were also increased. This observation clearly suggested that the rate of electrolysis is increased with the increment in applied voltage, resulting in the rise in volume of evolved H 2 and O 2 gases from co-electrolysis of CO 2 and seawater.
The Fig. 5 shows the GC analysis of the gaseous products collected from the microreactor at the applied electric potential (ψ) value of 2.5 V, 3 V, and 3.5 V, where oxy-hydrogen was collected from the outlet of microfluidic reactor 9,43 Table 1. Table 1. The FT-IR analysis was done to identify different organic species present in the aqueous product, collected from the microreactor operating at 2.5-3.0 V. The table shows the classification of the various functional groups based upon the peak positions in the FT-IR spectra reported in Fig. 4 Assigned Peak no. in Fig. 4 Peak position in Fig. 4 (cm −1 )

Assigned functional groups
Classification of the Organic compounds likely to present as end product 42 Vibration mode  Fig. S2B of ESI).
HPLC studies. The details of the HPLC analysis have been reported in Fig. S4 of the ESI. In the Fig. S4A in ESI, the peaks near the retention time (t R ) value of ~ 2.79 min and ~ 3.52 min. corresponds to formate and formic acid obtained from the microreactor, which further validates the FT-IR and GC-MS analysis results. The Fig. S4B (refer to ESI) shows the linear correlation between the volume of pure formic acid (V FA ) with corresponding the area under the curve (A FA ) as A FA = (1.1 × 10 8 ) V FA + 4.045. This suggests that CO 2 reduction and hydrogen evolution processes take place simultaneously at the cathode, as described by the proposed mechanism. In both the reactions, protons (H + ) are required and more H + was utilized towards H 2 evolution reaction.
Current density near electrodes. The Fig. 6A represents that the current density (J) in the microreactor increased with time (t) = 0-15 s before reaching a saturation value after t = 15-40 s, at the discrete electric poten- Table 2. Shows the details of GC-MS for the organic products obtained from microfluidic reactor operating at an electric potential value (ψ) ranging from 2.5 V to 3.5 V.   Fig. 6B shows that the average current (I avg -left y axis) and average current density (J avg -right y axis) of the microfluidic reactor increased linearly with the applied electric potential (ψ). It must be noted that since the distance between the two electrodes was close to unity the two curves almost overlap each other. An increment in current density signifies higher reaction rate along with H 2 gas evolution. The Fig. 6C shows a very similar trend as Fig. 6A for all the electric potential (ψ) values ranging from 2.5 V to 3.5 V when the readings are recorded in a single experiment by ramping up the voltage across the electrodes. In the microreactor, the system involves the formation of organic products at the downstream, and for error-free calculation of current density, the distance between the two electrodes was kept constant for all the experiments.
Microfluidic device prototype. We propose a microfluidic device (Patent number: 332899) enclosed inside a poly (methyl methacrylate) (PMMA) or acrylic glass. The Fig. 7 illustrates an envisioned membraneless microreactor-based device 44 which can be employed for the production of diverse organic species like formic acid, aldehydes, alcohols, formate salts, hydrocarbons, aliphatic and aromatic esters, primary and secondary alcohols using solar energy and in absence of any catalyst. The gaseous CO 2 and the salty seawater can be introduced through inlets, respectively of the microchannels embedded in the microreactor. The proposed microfluidic device will comprise of an integrated PV solar panel, four microreactor units (M1-M4), and electrical connections, all encased within the PMMA framework. This ergonomic design makes this solar energy-driven microfluidic device more versatile and efficient for synthesis of value-added organic compounds relative to the PDMS template-based microreactor proposed in this study.

Discussion
In summary, we designed a membraneless PDMS-based microreactor prototype comprising of built-in Cu electrodes linked to an external photovoltaic solar panel, and a T-shaped microchannel using the template molding technique for sequestration of greenhouse CO 2 gas. The naturally abundant CO 2 gas was introduced in to the microreactor and was converted into various value-added organic compounds using seawater as electrolyte and solar irradiation as energy source, thereby, emulating the photosynthesis process. As the integrated solar panel efficiently converts the sunlight into the electrical energy and the Cu electrodes, which are separated by a microscale distance, create a high-intensity electric field at a lower potential difference for the co-electrolysis of CO 2 and seawater, the proposed microreactor required a low operating electrical power for chemical reactions.  www.nature.com/scientificreports/ The incoming gaseous CO 2 and seawater after flowing into the region of high intensity electric field, undergoes rapid co-electrolysis in absence of any catalyst, produces nascent hydrogen, nascent oxygen and free radicals which in turn participates in the synthesis of diverse organic products ranging from aldehyde, formate salts, formic acid, primary or secondary alcohols, to short-chain hydrocarbons within the microreactor under continuous solar illumination. Importantly, by controlling the electric field intensity produced by the solar cells under solar irradiation, the reaction rate, and the amount of organic compound to be synthesized were regulated and optimized for industrial applications. The proposed proof-of-concept microfluidic reactor opens up the avenue for CO 2 sequestration and production organic compounds as it uses greenhouse CO 2 gas as reactant to synthesize a wide range of organic products. Furthermore, in near future, a very large-scale integration (µVLSI) of the these microreactors might help in scaling-up the production of diverse organic compounds in larger volumes.

Filtration of seawater. The seawater was vacuum filtered by passing it through ultrafine Nylon 66 Filter
Membrane using an oil-free vacuum pump for removal of undesirable solid contaminants, and the filtered seawater was used as electrolyte in the experiments. The pH and electrical conductivity of seawater after filtration was measured to be ~ 7.78 and ~4231 µS cm −1 , at room temperature, respectively. The filtered seawater was used in all the experiments, unless stated otherwise.
Co-electrolysis of seawater and CO 2 under natural solar irradiation. The template molding technique was used to fabricate a microreactor consisting of a two inlet-shaped microchannel with a diameter of ~ 500 μm and the copper (Cu) electrodes mounted in a position perpendicular to the microchannel. The detailed fabrication protocol of the microreactor is shown in the Fig. S1 in ESI of the manuscript. The PV panels were attached to the Cu electrodes and the solar panels were held for 2 h under solar light to stabilize the output voltages before starting the experiments. The solar panels were integrated directly inside the microchannel in the absence of any inverter with a power tolerance in the range of ± 3%. Thereafter, the output voltage was measured with the digital multimeter for finalizing the output voltage for performing the experiments. The electric field was varied by partly covering the solar panel with the aid of a black sheet. The applied electrical field potentials finalized for the tests were 2.5 V, 3 V and 3.5 V. In some of the experiments, solar PV panels were incorporated in series in order to verify the intensity of the applied electrical field. Seawater was injected into one of the microreactor inlets at a steady flow rate by a syringe pump (Harvard Apparatus, PHD 2000) and gaseous CO 2 was inserted into one of the inlets by maintaining a continuous supply of CO 2 from a commercial CO 2 gas cylinder. The gas-liquid mixture of CO 2 and seawater was flown through the microreactor to generate different organic chemicals under solar illumination. For each experiment, 5 ml of sea water was injected at a flow rate of Q w = 3 mL/min and the CO 2 gas was introduced at flow rates of Q g = 3 mL/min into the microchannel. Importantly, by controlling the field strength across the electrodes, the rate of organic compound synthesis were varied efficiently and the reaction intermediates and the organic products were collected downstream in a closed vial for further analysis.

Data availability
The datasets used and/or analyzed during the current study is available from the corresponding author on request.  www.nature.com/scientificreports/